Abstract

Background

Occurrence of blood and meat inclusions is an internal egg quality defect. Mass candling
reveals most of the spots, but because brown eggshell hampers selection in brown chicken
lines it has not been possible to eliminate the defect by selection. Estimated frequency
of blood and meat inclusions in brown layers is about 18% whereas it is 0.5% in white
egg layers. Several factors are known to increase the incidence of this fault: genetic
background, low level of vitamin A and/or D, stress or infections, for instance. To
study the genetic background of the defect, a mapping population of 1599 F2 hens from a cross of White Rock and Rhode Island Red lines was set up.

Results

Our histopathological analyses show that blood spots consist of mainly erythrocytes
and that meat spots are accumulations of necrotic material. Linkage analysis of 27
chromosomes with 162 microsatellite markers revealed one significant quantitative
trait locus (QTL) affecting blood spot and meat spot frequency. We sequenced a fragment
of a candidate gene within the region, ZO-2, coding for a tight junction protein. Nine polymorphisms were detected and two of
them were included in fine-mapping and association analysis. Fine-mapping defined
the QTL result. To further verify the QTL, association analyses were carried out in
two independent commercial breeding lines with the marker MCW241 and surrounding SNPs.
Association was found mainly in a 0.8 Mb-wide chromosomal area on GGAZ.

Conclusions

There was good agreement between the location of the QTL region on chromosome Z and
the association results in the commercial breeds analyzed. Variations found in tight junction protein ZO-2 and microRNA gga-mir-1556 may predispose egg layers to blood and meat spot defects. This paper describes the
first results of detailed QTL analyses of the blood and meat spots trait(s) in chickens.